The approach obviously doesn't work at all for unidirectional protocols. This approach also doesn't work when there are multiple equal cost paths, but with diff

The approach obviously doesn't work at all for unidirectional protocols. This approach also doesn't work when there are multiple equal cost paths, but with different MTUs. Which used to be more common and now tends to only happen when changing link technologies. The original sender needs not just the discovered MTU, but the original header information that may have been used for flow hashing.